Queued Service Observations - QSO-SNR mode

The QSO-SNR mode (using condition-driven exposure times with a SNR goal
rather than fixed exposure times) has been tested on ESPaDOnS and
MegaCam for a few semesters now. Following a
SAC
recommendation, the QSO-SNR mode will be the default operational
mode for ESPaDOnS and MegaCam, starting in 2017A.

This mode is NOT used for non-sidereal observations, very short
exposures of 30 seconds or less, or observations that require fixed
exposure times designed to ensure a specific spacing in time between
observations.

On ESPaDOnS, our experience also shows that the QSO SNR mode is not
optimal for (1) very low SNR goals (SNR of 30 and below), (2) targets with
unknown temperature and SNR wavelength goal different
from 730nm, (3) targets withtemperature < 3000K, and for (4)
targets which are intrinsically variable (with timescales ~ exposure).

To opt out of the SNR mode, PI must include a justification in their
proposal. All proposals, whether they will be performed with the SNR
mode or not, must request the number of hours needed to reach the
scientific goals under specific sky constraints, and justify the
exposure times and corresponding SNR. In particular, please ensure you
have added calculations from the relevant Exposure Time Calculator.

Please contact the qsoteam (qsoteam -=at=- cfht.hawaii.edu) if you
have questions or concerns.

1. What is the QSO-SNR mode?

In SNR mode, we integrate on the sky until the SNR requested by the PI
of a program is reached, or until the original exposure time is reached,
whichever happens first.

2. Is the SNR mode used on all instruments?

No. The SNR mode is not used on WIRCam or SITELLE. The SNR mode is used
on MegaCam and ESPaDOnS only.

3. Which programs are not suitable for the SNR mode?

Programs with very short exposures (30 seconds or less) are not worth
executing under SNR mode. MegaCam programs that use non-sidereal
tracking or guiding are not suitable for SNR mode. Programs that have
exposure times designed to ensure a specific cadence of observations, or
a specific time interval between exposures, are not suitable for SNR
mode.

On ESPaDOnS, our experience also shows that the QSO SNR mode is not
optimal for (1) very low SNR goals (SNR of 30 and below), (2) targets with
unknown temperature and SNR wavelength goal different
from 730nm, (3) targets withtemperature < 3000K, and for (4)
targets which are intrinsically variable (with timescales ~ exposure).

If you are writing a MegaCam or ESPaDOnS proposal for a project that
falls under those categories, please make sure you mention this in your proposal.

The SNR mode is the default mode for MegaCam and
ESPaDOnS, starting in 2017A. Unless the proposal has a justification for
not using the SNR mode, the program will be performed under the SNR
mode.

4. At night, is the SNR mode always enabled?

Programs are only executed if the current sky conditions meet (or
exceed) the constraints specified by the in PH2, whether the QSO-SNR
mode is used or not.

On MegaCam, if the measured Image Quality is the same as the one used to
compute exposure times in PH2, the full exposure time will be used. If
the Image Quality and other parameters are better than those used to
compute exposure times in PH2, a shorter exposure time will be used. (If
the measured IQ is higher than the one indicated in PH2, the program is
not executed.)

On ESPaDOnS, if the sky conditions are not stable (variable Image
Quality or variable extinction), the SNR mode is not used for polarimetric
observations.

5. How is the exposure time determined for ESPaDOnS SNR mode
observations?

ESPaDOnS has an exposure meter (EM) that picks up a few percent of
the object's light and records the flux in real time (see red line in
the figure below). The EM has been calibrated in such a way that we know
when we have reached the SNR (blue curve in the figure below) indicated
by the PI at the wavelength requested (green text and
horizontal line in the figure below).

The exposure is automatically stopped when the EM has reached the
target SNR (the blue curve intersects the horizontal green line), or
when the exposure time entered in PH2 has been obtained, whichever
happens first.

An exposure can not be lengthened.

6. How is the exposure time determined for MegaCam SNR mode
observations?

For MegaCam, an initial measurement of the sky conditions (Image
Quality, extinction, airmass) is performed, and those values are used in
the MegaCam ETC along with the indicated requested SNR. The exposure
time is calculated right before obtaining an observation.

Depending how the program is setup, the initial measurement can be
obtained with a "snap" short exposure or during the first exposure of a
dithering pattern. When a dithering pattern is used, individual
exposures can be shortened, or the number of exposures in the dithering
pattern can be decreased.

For a dithering pattern with a minimal number of exposures specified
by the PI, the exposure time computed at the beginning of the dithering
pattern is divided by the (minimal) number of exposures.

During the dithering pattern execution, the (quadratically) cumulated
signal-to-noise obtained so far is estimated, and the remaining
signal-to-noise goal (that would add quadratically up to the overall
signal-to-noise goal for the target) is used to compute the remaining
total exposure time. Again, this exposure time is divided by the number
of remaining mandatory exposures of the dithering pattern (if more than
two are remaining), or defines the exposure time of the next exposure
otherwise.

Two safeguards are in place to avoid runaway situations: the number n in
the chosen (L)DPn dithering pattern is considered as a maximum number of
exposures.

A maximum cumulated exposure time is also required, and in practice
should not exceed the exposure time given by the ETC under observing
conditions expected for the program by more than say 20%.

The software will ensure that those numbers are not exceeded even if
the signal-to-noise goal is not achieved.

This situation can happen in degrading observing conditions, and is a
good indication for the observer to switch to another program.

7. What if the flux unexpectedly drops during an exposure?

A MegaCam exposure taken under SNR mode will not be validated if the
requested SNR is significantly far from the requested SNR (due to the
seeing blowing up, or to the arrival of a cloud). Since the exposure
time was calculated for different sky conditions, the SNR will not be
reached. The exposure will be repeated (either right away, or once
conditions have improved).

An SNR mode ESPaDOnS exposure that doesn't reach its requested SNR will
not be validated (and will be repeated) if it differs by more than
~20%.

If conditions degrade significantly during a polarimetric sequence, the
SNR mode will be disabled and the polarimetric sequence restarted.

8. What are the advantages of the SNR mode?

If the IQ is better than the one used to calculate the exposure times,
then the SNR will be reached in a time shorter than entered in PH2. We
will then save time on the sky, which allows the QSO team to perform
more observations for the same program, or for other highly ranked
programs.

9. What happens to the time that is saved?

The time saved by using the SNR mode is not returned to any program in
particular. If an A-ranked program is performed in SNR mode, it is very
likely that at least part of the time saved on the sky will be used for
that program to hasten its completion. This is also true for B- and
C-ranked programs, but to a lesser degree.

10. If all my observations were performed using fewer hours than my
allocation, can I enter more observations to fill up my allocation?

Under some circumstances, and with prior approval of the QSO Team, PIs
may enter a few more observations than their allocation; this is used by
the QSO Team to add flexibility, and not necessarily to gather more
observations than initially planned.

11. Can my program be considered complete even if I haven't obtained all
my allocated hours?

Yes. If all the observations for a program have been obtained with fewer
hours than the allocated time, while still reaching the requested SNR,
then the program is considered complete.